The control of the series switching element of a clocked power supply unit is obtained by means of control pulses whose pulse-interval ratio is intended to lead to the most constant possible d.c. voltage delivered on the output side of the power supply unit. Towards such end, a regulating signal corresponding to the output voltage is derived and forwarded to a pulse length modulator in order to regulate the pulse-interval ratio of the control pulses in such a manner that the output d.c. voltage remains largely constant, independent of changes in load. Accordingly, electrical isolation of the regulation signal is required in order to ensure voltage isolation between the output circuit and the control circuit. In clocked power supply units optoisolators are employed to achieve such isolation because they facilitate the electrically separate transmission of d.c. voltage signals. A circuit arrangement operating a clocked power supply having a series switching element according to the principle described above is disclosed in German Democratic Republic Pat. No. 223,586. This prior circuit arrangement does not require a separate pulse-length modulator to modify the pulse-interval ratio of the control pulses because it is designed essentially as a Schmitt trigger circuit which is influenced directly by the regulating signal. In special operating cases, however, such as in starting and stopping of the power supply unit, additional control functions must be made available which ensure, for example, "soft" starting or slow discharge of the input capacitance of the power supply unit, which correspondingly influence the regulating circuit by their retroactions (i.e., reactions). This, however, renders the design of control circuits of the aforementioned kind rather complex.
In clocked power supply units, the circuit controlling the longitudinal logic element is predominantly capacitively loaded and in the interest of the highest possible limit frequency and lowest possible power input, should not delay, if possible, the charge processes of the capacitive load.